This invention relates to electromagnetic transducers and in particular to electromagnetic transducers of variable and nonspecific geometry.
Contemporary recording techniques enable the full dynamic range of musical performances to be captured, stored, and replayed at a later time. No longer esoteric technologies, consumers have enjoyed increasing access to these recording innovations in the form of stereo televisions, compact discs and digital tape. These improved recordings and transmission technologies, however, highlight the shortcomings existing technologies have in making the transfer between the representation of sound as an electrical/digital signal and as a mechanical vibration perceivable to the human ear. Furthermore, the miniaturization and portability of modern devices have placed additional demands on sound reproduction technology. The transfer between sound as an electrical signal and a mechanical vibration must now take place not only with improved fidelity, but within the limitations of confined spaces.
Conversion of an electrical signal to audible mechanical energy typically involves use of an electromotive force to vibrate a membrane. As the magnitude and/or direction of the electromotive force changes in synchronization with the audio signal, the membrane is excited, producing mechanical vibrations perceivable as sound. The magnitude of the electromotive force, the distance of the force to the moving element and the size and responsiveness of the moving element all influence the frequency response, dynamic range and distortion characteristics of the particular acoustical device corresponding technology. Previous technologies, such as cone speakers, ribbon speakers and planar magnetic speakers have suffered from fixed and distinct geometries that define the relationship between the origin of the electromotive force and the vibrating elements. These fixed geometries impose various limitations on the frequency response, performance characteristics, costs, and miniaturization potential of the given reproduction technology.
Cone speakers, for example, include a large permanent magnet placed at the rear of a vibrating cone element typically formed of paper. Cone speakers take an electronic audio signal from the amplifier and convert it into a magnetic field. The magnetic field interacts with the field of the permanent magnet to create a force that pushes and pulls the cone or diaphragm back and forth to vibrate the air and produce sound.
Although cone speakers have the advantage of highly directed sound, the frequency response of the cone is limited by the size of the vibrating element or cone. Thus, an individual cone speaker cannot reproduce sound efficiently over the full audio spectrum and several cones of different sizes are required to adequately cover the spectrum. In addition, the geometric relationship of the cone to the magnet permits operation in only one direction. The cone speaker is pushed forward by the permanent magnet and then snaps back. The return motion induces an electrical signal which is sent down the speaker cable back to the amplifier. In extreme cases, the listener hears these return signals as distortion. The predefined shape of the cone speaker also consumes a fixed volume of area and limits the locations in which the speaker may be placed. The paper cone further limits the environment in which the speaker can be placed.
Other speaker designs include planar magnetic speakers and ribbon speakers. The planar magnetic speaker consists of a flat, thin film diaphragm fitted with a voice coil and held taught in a metal frame. On the front of this frame is a large sheet of perforated material to which a row of vertically aligned strip magnets has been fastened. The coils are thus suspended in a stationary magnetic field. The coil moves back and forth within the field in synchronization with the music signal, vibrating the diaphragm and producing sound.
The geometric relationship of the vibrating diaphragm to the permanent magnets also imposes limitations on the planar magnetic speaker performance. For example the planar magnetic speaker is not a pushpull device. As the diaphragm is excited it is driven into the opposing magnetic field and away from its optimum position. Distortion and muddy tone can result. These speakers also suffer from an inability to drive them linearly with a correspondingly unsatisfactory dynamic range.
The fixed geometry of the planar magnetic speakers also imposes various manufacturing and cost difficulties. The planar speaker must remain flat. Also, in part because of the distance of the magnets from the diaphragm, the magnets used in assembly of such speakers are typically quite large in order to generate the relatively strong magnetic fields desired. This characteristic makes assembling the magnets in the speaker frame especially difficult. Once positioned in the frame, the magnets are usually secured in place. However, a strong enough attraction between magnets positioned on opposite sides of the diaphragm may occasionally free a magnet from its secured position during speaker assembly. The freed magnet then accelerates through the diaphragm, tearing it and completely destroying the speaker. The ferrous support frame exacerbates the difficulties of manipulating the large, powerful magnets and securing them within the frame. The large frame and magnets required also prohibit the miniaturization of the device for use in home and/or portable electronics devices.
A ribbon speaker contains a long narrow strip of conductive material usually formed of a piece of corrugated aluminum. The ends of this strip are connected to the audio amplifier and are physically anchored to the frame such that the strip is suspended within the magnetic field. As the audio signal passes through the foil, the strip moves within the magnetic field and produces sound.
The ribbon speaker does not reproduce low frequency sound. To reproduce sound at low frequencies, the opposing magnets must be moved so far apart that they no longer exert a sufficient magnetic field over the entire ribbon area. The ribbon driver is thus limited by the existing geometric relationship between the vibrating element and the electromotive force.
Alternate technologies use an electrostatic force instead of magnetic force as the electromotive element. One such design is the electrostatic speaker. The electrostatic speaker includes a diaphragm suspended in a rigid frame between two charged plates called stators. In an electrostatic speaker, as the amplifier produces a continuously varying AC voltage representing the audio information, the charge on the two stators undergoes a corresponding change. The constant charge diaphragm thus undergoes a change in attraction to and repulsion from the two stators as their polarization changes. This motion produces sound.
Electrostatic speakers can also suffer from limitations imposed by the fixed geometric relationship between the electromotive force and the vibrating element. Most electrostatic speakers are prone to arcing. Arcing is a condition of stress in which dielectric breakdown occurs and an electrical spark jumps between one stator and the diaphragm, burning a minute hole in the diaphragm. Overtime, arcing destroys the speaker.